1. Field of the Invention
The present invention relates to a die structure and a molding method for performing a transfer molding process of a molded product, having a small thickness and a relatively large area, at a low pressure and with a reduced production cycle time.
2. Description of the Related Art
An injection molding die provided with a heating element for generating heat through electric resistance heating is disclosed, for example, in Japanese Unexamined Patent Publication No. Hei 8-90624. In this disclosed example, a stamper, which is comprised of nickel having a thickness of about 0.3 mm and which generates heat through electric resistance heating, is attached to either one of a stationary die and a movable die, which are combined as a pair, by means of an inner circumferential holding member and an outer circumferential holding ring, to which both electrodes of an AC power supply delivering 100–500 A at 1–5 V are connected. Then, a polycarbonate resin is injected into a cavity that is formed by the stationary die and the movable die abutting each other, wherein the stamper is first energized to increase the temperature of the stamper surface to 130° C. instantaneously so that flowability and transferability of the resin is increased and, after that, the electric power is turned off, and the temperature of the stamper is reduced by the controlled temperature of the die so that a disk substrate is formed. Here, in order to prevent leakage current, an insulating layer, such as a ceramic coating, a sapphire coating and the like, is formed on a specular surface to which the stamper is attached and on a surface inside the hole penetrating the inner circumferential holding member.
Further, a laminating apparatus comprising a heating element that generates heat through electric resistance heating is disclosed in Japanese Unexamined Patent Publication No. Hei 10-315257. In this disclosed example, a molded material is disposed between film elements attached to opposing surfaces of an upper plate and a lower plate disposed in an opposing manner so that the both plates can move close to and away from each other and a molding space, in which the molded material is enclosed with a frame element disposed on the lower plate, is created by bringing the upper plate and the lower plate close to each other, air is evacuated from the molding space while the volume of the molding space is kept unchanged and, in this state, compressed air is introduced through air intake paths provided in the upper and lower plates so as to pressurize the molded material via the both film elements so that the molded material is separated from the upper and lower plates, through which channels for a coolant are provided and, at the same time, electric power is supplied to a heating plate, which is a rectangular thin plate made of stainless steel sandwiched by two film elements, to generate heat for heating the molded material for a predetermined time period. After that, the air is evacuated through one of the air intake paths so that the molded material is brought into intimate contact with either the upper or lower plate which is cooled by the coolant and, at the same time, the heating plate is de-energized so that the molded material is cooled to form a finished product.
Still further, another laminating apparatus comprising a heating element that generates heat through electric resistance heating is disclosed, for example, in Japanese Unexamined Patent Publication No. 2001-315202. In this disclosed example, a product is molded in a vacuum laminating apparatus, wherein an upper plate and a lower plate that can move close to or away from each other as well as frames attached to the upper and lower plates are pressed together to create a vacuum chamber therebetween and, then, in the vacuum chamber, an upper board and a lower board, through each of which channels for cooling water are pierced, are provided so that the upper board and the lower board can move close to and away from the upper plate and the lower plate by means of cylinders and, further, an upper film element and a lower film element are attached to the upper plate and the lower plate, respectively and, at the same time, heating plates comprised of rectangular thin plates of stainless steel are disposed between the upper board and the upper film element as well as between the lower board and the lower film element, respectively. More specifically, the upper plate and the lower plate are moved away from each other and the upper board and the lower board are put into intimate contact with the upper plate and the lower plate, respectively, by means of the respective cylinders and, at the same time, air is evacuated through each vent hole provided in each of the upper plate and the lower plate to press each heating plate and each film element against the upper board and the lower board, respectively. Then, in such state, after the molded material is placed on the lower film element, the upper plate and the lower plate are brought near to each other so that the frames attached to the upper and lower plates, respectively, abut each other and, after that, the air is evacuated through evacuation holes provided in each frame while the frames are pressed together. After that, each vent hole is changed into a mode to open the chamber to the atmosphere or a mode to supply compressed air to the chamber and a molding process is performed at a predetermined pressure, temperature and time period by pressurizing the molded material between the upper film element and the lower film element via the upper heating plate and the lower heating plate, respectively, and, at the same time, the upper heating plate and the lower heating plate are energized to generate heat. Then, the upper board and the lower board are moved close to each other by means of the respective cylinders so that the upper board and the lower board are put into intimate contact with the respective heating plates and, at the same time, the heating plates are de-energized to reduce the temperature to a predetermined value. Then, each evacuation hole is opened to the atmosphere and, at the same time, the air is evacuated through each vent hole to press each heating plate and each film element against the upper board and the lower board, respectively, and, further, the upper board and the lower board are brought into intimate contact with the upper plate and the lower plate, respectively, and, after that, the upper plate and the lower plate are separated from each other so that the molded product can be taken out.
Still further, an optical disk molding apparatus comprising a heating element that generates heat through induction heating is disclosed in Japanese Unexamined Patent Publication No. Hei 8-132498. According to this disclosed example, in a configuration in which a stamper supporting section having a small heat capacity and a temperature control section having a large heat capacity are disposed so that these elements can move close to and away from each other, when a resin material is injected, the temperature control section is moved away from the stamper supporting section and is induction heated by means of an induction coil so that a temperature drop of the resin material can be prevented and thus the quality of the optical disk can be improved and, on the other hand, when the resin material is solidified, the temperature control section is brought into contact with the stamper supporting section so that the resin material can be solidified quickly and thus the productivity of the optical disk can be increased.
Still further, in Japanese Unexamined Patent Publication No. Hei 10-34655, there is disclosed a technique for improving the transferability in which a die surface abutting on a stamper is covered with an electric resistance heating layer and said electric resistance heating layer is energized to heat the stamper.
However, among the prior art techniques described above, with regard to the injection molding method disclosed in Japanese Unexamined Patent Publication No. Hei 8-90624, as the electrodes are disposed on the inner circumferential holding member and the outer circumferential holding ring, the current density is not uniform in the direction from the inner circumferential holding member to the outer circumferential holding ring or, in other words, from the center to the outer circumference and, consequently, the heating temperature of the stamper is not uniform from the inner radius to the outer radius of the disk and there is a problem in that a disk substrate without distortion cannot be manufactured and uneven transfer may occur. Further, in this example, in which the stamper is brought into intimate contact with the die so as to heat the stamper, as the heat is absorbed by the die, which is temperature controlled to a set value lower than the heating temperature of the stamper, there is another problem in that it is difficult to increase the stamper temperature quickly and, further, it is also difficult to control the heating temperature of the stamper surface to an optimal value.
Similarly, with regard to Japanese Unexamined Patent Publication No. Hei 10-34655, in the configuration in which the stamper abuts on the specular surface covered with the electric resistance heating layer so as to heat the stamper, as the heat is absorbed by the specular surface, which is temperature controlled to a set value lower than the heating temperature of the stamper, there is also a problem in that it is difficult to increase the stamper temperature quickly and, further, it is also difficult to control the heating temperature of the stamper surface to an optimal value.
On the other hand, in the vacuum laminating apparatus disclosed in Japanese Unexamined Patent Publication (Kokai) No. Hei 10-315257, after the molded product is pressurized and heated via the film elements by compressed air, the air is evacuated through one of the air intake paths and the molded material is brought into intimate contact with the cooled upper or lower plate via the film elements so that the molded material can be cooled and molded. However, such cooling method in which the film elements and the heating plate are manipulated cannot be applied to the compression molding method in which the molded material is filled in the cavity so that it is compressed and molded under high pressure. Further, in the vacuum laminating apparatus disclosed in Japanese Unexamined Patent Publication No. 2001-315202, when the molded product is cooled, the upper board and the lower board are moved closer to the heating plate, but, if such cooling method is applied to an injection molding die, a complicated apparatus will be needed and the cost will be increased and, further, such method has a drawback that the molding cycle time is longer because the heating plate is cooled by moving the upper board and the lower board close to the heating plate. Thus, such method is not suitable for the molding apparatus that must operate in a small space, with reduced cost and with a reduced production cycle time.
Still further, in Japanese Unexamined Patent Publication No. Hei 8-132498, the temperature control section is moved away from the stamper supporting section and is induction heated by means of the induction coil. However, in the induction heating, as the temperature rises more slowly than in the resistance heating, there is a drawback that the molding cycle time is long. Further, as the induction heating uses an electromagnetic coil, a space for disposing the electromagnetic coil is needed. Also, as the induction heating uses eddy currents to generate heat, a circular heating element can be heated uniformly, but a substantially rectangular heating element cannot be heated uniformly. Further, in this example, when the resin material is solidified after the injection process is completed, the movable die is pressed so that the stamper supporting plate abuts on the temperature control section. In the actual injection molding process, however, as the time to apply the pressure to the resin material may often be delayed, optical disks of good quality may not be molded.